The present invention relates to a catalyst containing a carbide of at least one group VIB metal and phosphorous on an amorphous support, and to the use of this catalyst for hydrotreatment of hydrocarbon-containing, feeds with low sulphur contents.
The present invention also relates to the field of fuel for internal combustion engines, more particularly the manufacture of a fuel for a compression ignition engine from a novel hydrotreatment catalyst phase.
Gas oil cuts, whether from distillation or from a conversion process such as catalytic cracking, currently contain non negligible quantities of aromatic, nitrogen-containing, and sulphur-containing compounds. The current legislation of the majority of industrialised countries requires that fuel which can be used in engines must contain less than 500 parts per million by weight (ppm) of sulphur. In the very near future, this maximum quantity will be reduced to 350 ppm in about 2000 and to 50 ppnm in about 2005 for the member states of the European Community. Regarding the amount of polyaromatic compounds in gas oils, this may be reduced to a very low value (of the order of 1% to 2%) from 2005. In this context, hydrogenation of the polyaromatics contained in gas oil cuts is thus increasing in importance, because of the new sulphur and aromatic compound limits in this type of fuel.
Desulphurisation is generally carried out under conditions and using catalysts which are not capable of simultaneously carrying out hydrogenation of the aromatic compounds. Thus a first treatment of the cut must be carried out to reduce the sulphur content followed by a second treatment to hydrogenate the aromatic compounds contained in the cut. This second step is generally carried out by bringing the cut, in the presence of hydrogen, into contact with a catalyst generally based on a noble metal. However, because the desulphurisation process can never completely eliminate the sulphur-containing and nitrogen-containing compounds, the catalysts used must be able to operate in the presence of such compounds, which are powerful inhibitors of the activity of noble metals. It is thus important to seek out active phases having good thio-resistant properties. The aim of the present invention is thus to discover a novel catalyst based on a carbide which could be substituted for the noble metals used by the skilled person.
The use of massive or supported group VIB carbides as hydrotreatment catalysts for certain reactions on a model or actual feed has already formed the subject matter of prior art publications (S. T. Oyama et al., in Catal. Today, 15 (1992) pp. 179-200, or App. Catal., 168 (1998), pp. 219-228 and App. Catal. A., 134 (1996), pp. 339-349).
The Applicant has discovered that surprisingly, the introduction of a quantity of phosphorous can substantially improve the activity of a catalyst containing at least one carbonized group VIB metal on an amorphous oxide support, preferably alumina or silica-alumina. Further, the activity of the catalyst is better if during preparation of the catalyst, a heteropolyanion type compound containing at least one group VIB element, phosphorous and group VIB elements introduced by any precursor is preferentially used. The catalyst can optionally contain at least one element from group VIII of the periodic table. Such a catalyst can advantageously carry out hydrodesulphurisation and hydrogenation of aromatic compounds in hydrocarbon-containing feeds containing sulphur-containing compounds.
The invention also concerns the use of said catalyst for treating hydrocarbon-containing cuts containing sulphur and aromatic compounds and more particularly gas oil cuts from distilling crude oil and a variety of conversion processes such as cuts known as xe2x80x9ccycle oilsxe2x80x9d from catalytic cracking processes. The catalyst of the present invention can be used for desulphurisation and hydrogenation of hydrocarbon-containing cuts. The feed which can be treated using the process of the invention has sulphur contents of less than 2000 ppm by weight, preferably 0.01 to 500 ppm by weight. However, this catalyst can also be suitable for any process aimed at hydrogenating all or a portion of the aromatic compounds of a feed containing traces of sulphur-containing compounds, such as hydrogenation of aromatic compounds in edible oils and in solvents.
The catalyst of the present invention generally comprises, in weight % with respect to the total catalyst weight:
0.1% to 30% of a carbide phase containing at least one group VIB element with formula MxCy where M is at least one group VIB element and the ratio y/x is in the range 0.75 to 0.25;
01% to 10% of phosphorous; and optionally:
0 to 10% of at least one metal from group VIII of the periodic table.
Thus the catalyst comprises phosphorous and a group VIB metal, such that the preferred P/VIB metal mole ratio is in the range 0.05 to 1.2, more preferably in the range 0.08 to 0.55.
The catalyst is characterized in that the carbide phase is in the form of small particles with a size of less than 80 xc3x85, preferably less than 50 xc3x85 and more preferably less than 30 xc3x85.
The catalyst of the present invention can be prepared using any method which is well known to the skilled person. Preferably, the catalyst of the present invention can be obtained using the following steps:
a) impregnating a solution into an amorphous oxide matrix, said solution containing at least one group VIB element, phosphorous and optionally a group VIII element. Preferably, a salt of a heteropolyanion containing at least one group VIB element and phosphorous and optionally at least one group VIII element more generally with formula AxByCzOn is used, where A is at least one group VIB element, B is a group VIII element, C is phosphorous and O is oxygen, where the ratios z(x+y) can be in the range 0.05 to 1.2, preferably in the range 0.08 to 0.55,
b) optionally, drying;
c) optionally, activating the catalyst in an oxidising or neutral mixture,
d) optionally, carrying out a reduction step;
e) carbonization with a hydrocarbon;
f) optionally, passivating in an inert gas plus oxygen.
Impregnation step a) can be carried out using any method which is well known to the skilled person. The components constituting the catalyst can be introduced separately into the catalyst, in successive addition steps using solutions of one or more elements, or simultaneously using a common solution of the elements. When a plurality of impregnation steps are carried out to produce the catalyst, drying or activation (calcining or reduction) steps can be carried out between two successive impregnation steps.
Drying step b) can be carried out using any method which is well known to the skilled person, at a maximum temperature of 150xc2x0 C.
Activation step c) consists of calcining in a neutral or oxidising mixture using any method which is well known to the skilled person at a temperature of more than 100xc2x0 C. and less than 900xc2x0 C.
Reduction step d) consists of reduction in pure hydrogen or hydrogen mixed with an inert gas (for example argon) at a temperature of more than 100xc2x0 C. and less than 900xc2x0 C.
Carbonization step e) is carried out in two parts. A first part consists of placing the catalyst in an inert atmosphere (for example under argon) up to temperatures of 400xc2x0 C. or 500xc2x0 C. depending on the group VI metal used. The second part (the carbonization step proper) consists of carbonizations preferably using a programmed temperature profile in a hydrocarbon/hydrogen mixture (for example a mixture of 20% methane or any alkane, alkene or alkyne used pure or as a mixture and with the complement to 100% of hydrogen) to final temperatures of 677xc2x0 C. to 850xc2x0 C. Depending on the metal used, a final constant temperature stage is necessary to obtain a catalyst with good catalytic properties.
Step f) is a passivation step. Since this type of catalyst can be pyrophoric, a supplemental step is then necessary to avoid total oxidation of this catalyst. This step is generally carried out at ambient temperature (25xc2x0 C.) by placing the catalyst under an inert gas then adding a low partial pressure of oxygen to that gas (1%) for periods of 1 to 15 hours.
The amorphous oxide matrix is selected from transition aluminas, silicas and silica aluminas and mixtures thereof. This type of support has a specific surface area, determined using techniques which are known to the skilled person, in the range 100 to 600 m2/g, preferably in the range 150 to 500 m2/g. The amorphous oxide support can be used in the form of a powder or pre-formed in the form of beads or extrudates.
Sources of group VIB elements which can be used are well known to he skilled person. As an example, preferred sources of molybdenum and tungsten which are used are oxides and ammonium salts are used such as ammonium molybdate, ammonium heptamolybdate and ammonium metatungstate. Preferably, salts of heteropolyacids of group VIB metals such as phosphomolybdic acid or phosphotungstic acid are used.
The precursors of group VIII metals which can be used are well known to the skilled person. As an example, for non noble metals, nitrates, sulphates, phosphates, halides, for example chlorides, bromides and fluorides, or carboxylates, for example acetates and carbonates, are used. For the noble metals nitrates are used when they exist, halides, for example chlorides, nitrates, acids such as chloroplatinic acid, or chloroiridic acid, alkali metal chlorometallates, chloro-or hydroxo-amminated complexes, or oxychlorides such as ammoniacal ruthenium oxychloride. It is also possible to use soluble co-ordination complexes in organic solvents, such as acetylacetonate complexes. It is also possible to use carbonyl complexes.
Preferably, the group VIII metal is selected from non-noble metals and preferably from nickel and cobalt.
The hydrocarbons used for the carbonization step can be selected from alkanes, alkenes, alkynes, aromatic compounds or any other hydrocarbon-containing compound which is well known to the skilled person for its carbonizing properties. These hydrocarbons can be mixed with an inert gas or with hydrogen.
The hydrodesulphurisation process of the invention is generally carried out at temperatures of 100xc2x0 C. to 400xc2x0 C., preferably 150xc2x0 C. to 380xc2x0 C. The operating pressure is generally 0.1 to 30 MPa, preferably 1 to 20 MPa. The space velocity, expressed as the volume of liquid feed treated per volume of catalyst per hour, is generally 0.1 to 20 h1xe2x88x92. The hydrogen/feed ratio used is expressed as the volume of hydrogen measured under normal conditions per volume of liquid feed; it is generally 50/1 to 2000/1.
The feeds used generally contain at least 10% of aromatic compounds and less than 2000 ppm of S. They may be kerosines or gas oils from atmospheric distillation or feeds originating from refining processes such as catalytic cracking, coking, visbreaking, and hydroconversion of residues.